High-powered transistors used for amplifying ultra-high frequency (UHF) and microwave frequencies often have low input and/or low output impedances which necessarily require matching network components to be placed as close as possible to the transistor in order to achieve reasonable power bandwidths. Typically in order to connect a radio frequency (RF) power transistor in an actual circuit, the flange of the transistor is mechanically screwed to a heat sink while the base, emitter and collector tabs of the transistor are soldered to a printed circuit board on which the necessary matching network components are placed. Thus soldering is generally used as both the electrical and mechanical fastening technique for such transistors.
In circuit applications which do not permit the transistor to be driven continuously, such as the cyclical operation of power amplifiers used for cellular communications traffic, thermal cycling stress and associated mechanical creep can lead to early failure of solder joints, especially in high current applications having small joint cross-sections, and especially where the material connected by soldering have different co-efficients of thermal expansion. The latter condition is prevalent in situations where solder is used as the method of joining a power transistor or other high-powered device to a printed circuit board trace.
The above-referenced conditions which can lead to thermal cycling stress and mechanical creep are particularly present for solder joints between the collector output tab of high power RF transistors, especially those that use nickel steel (known as Alloy 42) for the output tab, and an associated circuit board trace and the tabs of the closely spaced matching capacitors. Typical lifetimes of such solder joints when cycled at full RF power are from two to four months in cellular communication applications, and somewhat longer if power output is reduced. Identical amplifiers using the same transistors but operating on a continuous basis do not exhibit such solder joint failures. Unfortunately, in many applications, including such cellular communication signal channel amplifiers, energy costs are significant and as a result, the amplifiers are operated on a cyclical need basis dependent on the voice traffic.
A prior art fastening technique which can withstand the thermal cycle stresses and mechanical creep associated with intermittent high power use is known as brazing. This process uses high melting temperature materials, such as silver alloys, for connecting two electrically conductive objects which have still higher melting temperatures. This technique is commonly used in air conditioning and heating applications. However, the temperatures needed for conducting the brazing operation are typically in excess of 800 degrees F. (in excess of 426 degrees C.). These temperatures cannot be tolerated by power transistors nor by typical printed circuit boards and related components. In addition, other prior art electrical connection techniques which are commonly used in automotive and home wiring make use of mechanical fastening methods such as bare wire connected with screws, crimped connections, posts and the like. Such techniques are not satisfactory for preventing connection failures due to thermal stress and associated mechanical creep encountered in the high-powered transistor applications. They are also unsatisfactory for high frequency applications (typically above 400 MHz) since the wire used in such techniques introduce significant inductance into the overall circuit.
The present invention provides a solution for mounting such high-powered transistors by use of a transistor mounting clamp assembly.